The process of resistance welding involves the positioning a workpiece to be welded between a pair of electrodes through which electrical current is delivered from a source thereof. The electrodes are clamped under pressure to squeeze the workpiece therebetween so that electrical current is delivered from one electrode through the workpiece to the other electrode at the point of contact between the electrodes and the workpiece. Heat generated by the resistance encounted by the current passing through the parts of the workpiece to be joined melts the contact faces of the part, thereby melting the parts to create a weld. In some cases, after the weld joint between the parts has reached a sufficiently high temperature, electrical current flow is terminated and the clamping pressure is maintained for a prescribed time interval to unite the parts. In other cases, such as spot welding, the parts need not remain clamped after current flow is terminated.
In order to adapt pulse power type welding apparatus for use in various applications, control systems have been devised which allow selective control of each sequence in a welding cycle, i.e., total clamping time, welding time, holding time, after termination of welding current, as well as the magnitude of current delivered to the electrodes. These prior art systems employed mechanical timers and potentiometers arranged in an analog circuit to control the various process variables. Systems of this type were undesirable, however, in that the process parameters could not be selected and controlled with the repeatable precision required in many applications.
In order to deliver additional current to the workpiece in some applications, a three phase AC power supply is employed to increase the frequency of welding pulses. The use of a three phase AC power supply substantially complicates the system for controlling the various process parameters.
Accordingly, it is a primary object of the present invention to provide a solid state control system employing a mircocomputer for accurately and reliably controlling a resistance-type pulse welding apparatus employing a three phase power supply.
A further object of the present invention is to provide welding apparatus and a control system therefor as described above which is highly flexible in terms of its adaptability to various welding applications.
A further object of the invention is to provide a control system as described above in which process parameters are precisely selected and stored in the memory for use in controlling the welding apparatus.
A still further object of the invention is to provide a control system as described above which includes a sensing circuit for protecting the level of line voltage and inhibiting the operation of the mircocomputer when the line voltage falls below a prescribed level thereof.
A further object of the invention is to provide a three phase power supply for a welding system as described above which substantially reduces the size and number of components for the circuit.
Another object of the invention is to provide a three phase power supply as described above which employs a welding transformer having three separate primary coils supplied by each phase of the power supply and connected to a common ground terminal.
These, and further objects of the invention will be made clear or will become apparent during the course of the following description of a preferred embodiment of the present invention.